Part of the computer system of the Large Hadron Collider (LHC) was hacked into as the world's most powerful physics experiment got under way.
A group calling itself the 'Greek Security Team' hacked into a computer connected to the system last Wednesday.
A spokesman for Cern, the lab that houses the LHC, said the hackers put up a message on the facility's website.
No harm was done but the incident has highlighted the need for security in the LHC's network, the spokesman said.
The hackers had targeted the computer network of the Compact Muon Solenoid Experiment (CMS), a huge detector that analyses data from the particle accelerator.